Vinylidene fluoride (VDF) copolymers comprising recurring units derived from trifluoroethylene (TrFE) monomer have been used extensively in the electronics packaging market due to their ease of processing, chemical inertness and attractive ferroelectric, piezoelectric, pyroelectric and dielectric properties.
As is well known, the term piezoelectric means the ability of a material to exchange electrical for mechanical energy and vice versa and the electromechanical response is believed to be essentially associated with dimensional changes during deformation or pressure oscillation. The piezoelectric effect is reversible in that materials exhibiting the direct piezoelectric effect (the production of electricity when stress is applied) also exhibit the converse piezoelectric effect (the production of stress and/or strain when an electric field is applied).
Ferroelectricity is the property of a material whereby this latter exhibits a spontaneous electric polarization, the direction of which can be switched between equivalent states by the application of an external electric field.
Pyroelectricity is the ability of certain materials to generate an electrical potential upon heating or cooling. Actually, as a result of this change in temperature, positive and negative charges move to opposite ends through migration (i.e. the material becomes polarized) and hence an electrical potential is established.
It is generally understood that piezo-, pyro-, ferro-electricity in copolymers of VDF with TrFE is related to a particular crystalline habit, so called beta-phase, wherein hydrogen and fluorine atoms are arranged to give maximum dipole moment per unit cell.
Copolymers comprising recurring units derived from vinylidene fluoride and trifluoroethylene are typically provided as semicrystalline copolymers which can be shaped or formed into semicrystalline, essentially unoriented and unstretched, thermoplastic film or sheet or tubular-constructed product via well known processing methods such as extrusion, injection moulding, compression moulding and solvent casting.
Nevertheless, more recently, developments of thin film electronic devices and/or assemblies of ferroelectric polymer layers in three-dimensional arrays for increasing e.g. memory density have called for different processing techniques, requiring notably ability of the polymer to be patterned according to lithographic techniques and/or for layers there from to be stacked with annealing treatment on newly formed layer not affecting previously deposited layers.
Within this scenario, thus, cross-linking (elsewhere referred to as ‘curing’), which is one of the most known techniques in polymer science to stabilize shape and fix structures, has been the technique of choice for accessing these needs.
Solutions have thus been proposed for conferring to VDF-TrFE copolymers cross-linking or curing ability. Among those solutions, use of azide-containing coupling agents, because of their ability of inserting into carbon-hydrogen bonds under thermal or UV treatment, and yet of their relative robustness, has been considered. So, VAN BREEMEN, A. J. J. M., et al. “Photocrosslinking of ferroelectric polymers and its application in three-dimensional memory arrays”. Appl. Phys. Lett. 2011, vol. 98, p. 183302. and US 2007/166838 (PHILIPS ELECTRONICS NORTH AMERICA CORPORATION) discloses a photolithography process designed to provide access to three-dimensional memory arrays, said process involving the photocrosslinking of VDF-TrFE polymers using as cross-linking agent azide or azo-compounds, including notably 2,6-bis(4-azidebenzylidene)-4-methylcyclohexanone and other analogous compounds.
Similarly, WO 2005/064705 (KONINKLIJKE PHILIPS ELECTRONICS N. V.) Jul. 14, 2005 discloses patterning by means of photolithography of fluorinated ferroelectric polymer layers, such as those derived from VdF-TrFE (random) copolymers, by addition of a photosensitive cross-linker, such as, e.g., a bis-azide, to a fluorinated polymer spin-coat solution. Nevertheless this document is deprived of any specific description of suitable bis-azide derivatives.
Nevertheless, in these approaches, use is made of non-fluorinated bis-azides, whose dispersability in the curable polymer compound is rather difficult because of inherent incompatibility of the hydrogenated moieties within the fluoropolymer matrix, so that curable mixture with uneven distribution of crosslinking agent can typically obtained, unless using severe mixing conditions which might elsewhere cause premature reaction/decomposition of the azide reactive moieties. Further, curable compounds with uneven distribution of crosslinking agent might provide, by curing, materials with regions having different crosslinking densities, which, as a consequence, might also possess unequal piezo-, pyro-, ferro-electricity properties, due to different microstructure.
There is thus still a need in the art for VDF/TrFE polymer crosslinkable composition which can undergo crosslinking under thermal or UV exposure conditions, yielding a uniformly cured material possessing improved thermal stability and which still maintains outstanding piezoelectric, ferroelectric, pyroelectric and dielectric properties.
On the other side, the azide-containing curing agents are known in the art, in particular for effecting curing of fluoroelastomers.
Thus, US 2010324222 (DUPONT PERFORMANCE ELASTOMERS) Dec. 23, 2010 discloses curable fluoroelastomer compositions comprising a VdF- or TFE-based fluoroelastomer comprising cure site monomers having a cure site selected from nitrile and alkyne groups and a curing agent having formula N3(Y)p—(CH2)n—R—(CH2)m—(Y)pN3, wherein Y is SO, SO2, C6H4, or CO, p is 0 or 1, n, m are independently 1 to 4, and R is selected from the group consisting of i) a C3-C10 fluoroalkylene group, ii) a C3-C10 fluoroalkoxylene group, iii) a substituted arylene group, iv) an oligomer comprising copolymerized units of vinylidene fluoride and perfluoro(methyl vinyl ether), v) an oligomer comprising copolymerized units of vinylidene fluoride and hexafluoropropylene, vi) an oligomer comprising copolymerized units of tetrafluoroethylene and perfluoro(methyl vinyl ether), and vii) an oligomer comprising copolymerized units of tetrafluoroethylene and a hydrocarbon olefin. Non limitative examples of such curing co-agents are notably N3—CH2CH2—(CF2)6—CH2CH2—N3, N3—CH2CH2(CF2)4—CH2CH2—N3 and N3—CH2CH2-poly(VdF-co-PMVE)—CH2CH2—N3.